An MS twin registry will be instituted by ascertainment of large numbers of twins with Multiple Sclerosis by various means. Symptomatic twins will be characterized by the probability of the diagnosis using the Schmacher criteria, by the dates of clinical events, by disability using the Kurtzke scale, by zygosity, and by various non-medical attributes. Unambiguously discordant pairs will be studied using standard case-control methods, taking advantage of the twins' comparability, motivation, and common knowledge, including their ability to rank themselves by the proximity, intensity and sequence of past events and exposures. Blood for HLA and serology and dermatoglyphic specimens, will be taken. Other pairs will be used for other comparisons, and mechanisms will be instituted to permit solicitation of selected pairs for other studies.